Image display device and head mount display

ABSTRACT

Provided are an image display device and HMD, which can make an observer observe an image with reduced luminance nonuniformity and reduced color nonuniformity, while making the image bright by reducing the degree of diffusion with a constitution employing a means which diffuses incident light in one direction. In the image display device, light from a light source is diffused in one direction with the diffusing means and is guided to a display element, and image light emitted from the display element is guided to an optical pupil through a lens. The image display device with the constitution satisfies the expression Hw/f≦tan 1′Kw/(f+δ), wherein Hw is the aperture width of pixels of the display element in the diffusing direction of the diffusing means, Kw is the maximum pitch of recesses and protrusions in the diffusing direction of the diffusing means, f is the focal length of the lens, and δ is the distance between the display surface of the display element and the diffusing surface of the diffusing means.

TECHNICAL FIELD

The present invention relates to an image display device for providingan observer with an image displayed on a display element as a virtualimage, and a head mount display (also referred to as “HMD” hereinafter)equipped with the aforementioned image display device.

BACKGROUND ART

An image display device having been invented so far to provide anobserver with an image displayed on an LCD or similar display element asa virtual image is exemplified by the invention disclosed in the PatentLiterature 1. In the image display device of Patent Literature 1, thelight emitted from an LED array is diffused by a diffusing plate and isled into the LCD. The image light from the LCD is then led to an opticalpupil through optical elements such as a prism or lens. When the pupilof the observer is set to the position of the optical pupil, theobserver is allowed to see the image displayed on the LCD.

Incidentally, to facilitate observation of an image (virtual image) bythe observer, it is essential to brighten the image while expanding theoptical pupil. In this case, if the incident light is diffused in alldirections by a diffusing plate, the brightness of the light will bereduced, even if the optical pupil can be expanded in all directions.Thus, it is very effective to use a diffusing plate that diffusesincident light in one direction (unidirectional diffusing plate),instead of a diffusing plate that diffuses the incident light in alldirections. To be more specific, the unidirectional diffusing plate isused to diffuse the incident light in one direction and to form a bigoptical pupil in one direction, whereby easier observation of the imageby an observer is ensured. Further, the light from the light source isconverged in the direction perpendicular to the aforementioned directionof diffusion so that the observer is provided with a brighter image.

CITATION LIST Patent Literature

-   Patent Literature 1: Unexamined Japanese Patent Application    Publication No. Hei 11 (1999)-326821

SUMMARY OF INVENTION Technical Problem

FIG. 17 a is a cross sectional view representing an example of theconfiguration of a unidirectional diffusing plate 101. Thelight-emitting surface 101 a as the diffusing surface of theunidirectional diffusing plate 101 is formed of recesses andprotrusions. In this structure, when light has entered theunidirectional diffusing plate 101 from a light incoming surface 101 b,light is not emitted because of total reflection from the portion of thelight-emitting surface 101 a having a steeper inclination of therecesses and protrusions, for example, with the result that the shade Pcorresponding to the aforementioned portion wherein light is not emittedwill appear in the light emitted from the unidirectional diffusing plate101.

Thus, if the unidirectional diffusing plate 101 is applied to aconventional image display device, the shade P of the unidirectionaldiffusing plate 101 will be superimposed on the observed image (virtualimage) Q, as shown in FIG. 17 b, and luminance nonuniformity and colornonuniformity will appear to deteriorate the quality of the observedimage Q. This phenomenon is likely to occur when the aperture width ofeach pixel of the display element is greater than the widthcorresponding to the resolution (e.g., one minute in terms of viewingangle) of the human eye, and when the unidirectional diffusing plate 101is located close to the display element. To be more specific, FIG. 18 isan enlarged view of the display region of the display element (e.g.,LCD). If there is an excessive aperture width Hw of each pixel A in thediffusing direction, the shade P superimposed on the pixel A can beidentified, with the result that the quality of the observed image willbe deteriorated. To avoid this problem, if the unidirectional diffusingplate 101 is located far away from the display element, the lightutilization efficiency will be reduced and the observed image Q will bedarkened. This is not recommended.

If the diffusion of the unidirectional diffusing plate 101 is excessive,the image will be darkened. The degree of diffusion is preferablyreduced to ensure a bright image to be observed. However, reduction inthe degree of diffusion signifies an increase in the size of the pitchof the recesses and protrusions in the diffusing direction. This is morelikely to cause luminance nonuniformity and color nonuniformity to beproduced by the shade of the unidirectional diffusing plate 101.

As disclosed in the Patent Literature 1, if the surface light sourcesuch as an LED away is used as a light source for illuminating thedisplay element, a greater amount of diffusion will be required toreduce the luminance nonuniformity of the surface light source, and theimage will be darkened. This is not recommended.

The present invention is intended to solve the aforementioned problems.It is accordingly an object of this invention to provide an imagedisplay device including a means for diffusing the incident lightunidirectionally, wherein the observer is supplied with a high-qualityimage with reduced luminance nonuniformity and color nonuniformity,whereas the degree of diffusion of the diffusing means is reduced toprovide a bright image. The present invention also intends to provide ahead mount display equipped with the aforementioned image displaydevice.

Solution to Problem

An image display device of the present invention is an image displaydevice comprising: a point light source; a diffusing means comprising adiffusing surface in which recesses and protrusions are formed in onedirection with a plurality of pitches, for diffusing light travelingfrom the light source by the diffusing surface in the one direction andemitting the diffused light; a display element comprising a plurality ofpixels and a light-shielding section formed around the plurality ofpixels, for displaying an image by modulating light traveling from thediffusing means with the plurality of pixels; and an ocular opticalsystem for guiding light of an image traveling from the display elementto an optical pupil, to make an observer observe a virtual image of theimage at the optical pupil. The image display device is characterized bysatisfying the following conditional expression (1), where Hw is anaperture width of the pixels of the display element in a diffusingdirection of the diffusing means, Kw is a maximum pitch of the recessesand protrusions in the diffusing direction of the diffusing means, f isa focal length of the ocular optical system, and 3 is a distance from adisplay surface of the display element to the diffusing surface of thediffusing means.

Hw/f≦tan 1′≦Kw/(f+δ)  (1)

In the image display device of the present invention, it is preferablethat an aperture ratio Hw/Hp of the pixels of the display element in thediffusing direction of the diffusing means satisfies the followingconditional expression (1A), where Hp is a pitch of the pixels of thedisplay element in the diffusing direction of the diffusing means.

0.3≦Hw/Hp≦0.8  (1A)

In the image display device of the present invention, it is preferablethat the diffusing means comprises a diffusing surface in which recessesand protrusions are formed in one direction with a plurality of pitchesranging from 1 μm to 20 μm.

In the image display device of the present invention, the light sourcemay comprise light-emitting diodes emitting light of three colors ofred, green, and blue, respectively, and the light-emitting diodes may bearrayed in the diffusing direction of the diffusing means.

In the image display device of the present invention, the light sourcemay comprise two sets of light-emitting diodes for emitting red, green,and blue light, respectively, and the light-emitting diodes of eachcolor may be arranged symmetrically with respect to an optical axis ofthe ocular optical system, where the optical axis is an axis opticallyconnecting a center of a display region of the display element and acenter of the optical pupil.

In the image display device of the present invention, it is preferablethat the distance δ from the display surface of the display element tothe diffusing surface of the diffusing means is set such that adifference in visibility between a virtual image formed by the ocularoptical system of the diffusing means and a virtual image of an imagedisplayed on the display element is 10 diopters or less.

In the image display device of the present invention, the diffusingmeans may be arranged such that the diffusing direction intersects witha long-side direction of a rectangular display region of the displayelement.

In the image display device of the present invention, it is preferablethat a diffusion degree of the diffusing means in the diffusingdirection is ten or more times larger than a diffusion degree of thediffusing means in a direction perpendicular to the diffusing direction.

In the image display device of the present invention, it is preferablethat an opposing surface to the diffusing surface in the diffusing meansis a non-diffusing surface, and that the diffusing means is arrangedsuch that the diffusing surface faces the display element.

In the image display device of the present invention, it may be furthercomprise a light-converging means for converging light traveling fromthe light source, in a direction perpendicular to the diffusingdirection of the diffusing means and for guiding the light to thediffusing means, wherein the light source and the optical pupil may bearranged at conjugate positions along a direction perpendicular to thediffusing direction of the diffusing means.

In the image display device of the present invention, it is preferablethat the light-converging means has no-optical-power ornegative-optical-power in a direction parallel with the diffusingdirection of the diffusing means.

In the image display device of the present invention, it is preferableto satisfy the following conditional expression (2), where Vw is anaperture width of the pixels of the display element in a directionperpendicular to the diffusing direction of the diffusing means.

tan 1′≦Vw/f  (2)

In the image display device of the present invention, it may furthercomprise an optical-path bending member for bending an optical path fromthe light source to the display element, wherein the diffusing means maybe arranged in an optical path between the optical-path bending memberand the display element.

In the image display device of the present invention, the optical-pathdifference bending member may comprise a refracting surface and areflecting surface. After the refracting surface refracts lighttraveling from the light source and the reflecting surface reflects thelight, the refracting surface may refract the light again to guide thelight to the diffusing means.

In the image display device of the present invention, the ocular opticalsystem may comprise a hologram optical element of a volume phase typeand a reflection type, and the hologram optical element may diffract andreflect light of an image traveling from the display element to guidethe light to the optical pupil.

In the image display device of the present invention, it is preferablethat the hologram optical element has axi-asymmetric positive opticalpower.

In the image display device of the present invention, it is preferablethat the diffusing direction of the diffusing means is almost parallelwith a direction perpendicular to an optical-axis incident surface ofthe hologram optical element, where the optical axis is an axisoptically connecting a center of a display region of the display elementto a center of the optical pupil.

In the image display device of the present invention, it is preferablethat the light source comprises a light-emitting diode, and a wavelengthat which a diffraction efficiency of the hologram optical element ismaximum and a wavelength at which an intensity of light emitted from thelight source are almost same.

In the image display device of the present invention, the ocular opticalsystem may comprise a first transparent substrate for totally reflectinglight of an image from the display element on a inside thereof; to guidethe light to the optical pupil, and for transmitting outside light toguide the outside light to the optical pupil.

In the image display device of the present invention, the ocular opticalsystem may further comprise a second transparent substrate for cancelingrefraction of the outside light caused in the first transparentsubstrate.

A head-mounting display of the present invention is a head-mountingdisplay characterized by comprising: the above-described image displaydevice; and a supporting means for supporting the image display deviceat a front of an eye of an observer.

Advantageous Effects of Invention

When the conditional expression (1) is met, the pitches of the recessesand protrusions on the diffusing surface of the diffusing means can beenlarged to reduce the degree of diffusion, whereby the observer issupplied with a bright image. Further, the aperture width of the pixelsof the display element in the diffusing direction can be reduced, andthe greater part of the shade of the recesses and protrusions on thediffusing surface can be superimposed on the light-shielding section ofthe display element. Moreover, for the shade superimposed on the pixelsof the display element, the aperture width of the pixels is smaller thanthat corresponding to the resolution (corresponding to one minute interms of viewing angle) of the human eye. This makes it more difficultfor the observer to identify the shade through the pixels. To be morespecific, when the conditional expression (1) is satisfied, the observeris supplied with a high-quality image with reduced luminancenonuniformity and color nonuniformity resulting from the shade of therecesses and protrusions of the diffusing means, while the degree ofdiffusion of the diffusing means is reduced to provide a bright image.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is an explanatory diagram schematically representing variousforms of parameters in an image display device as an embodiment of thepresent invention;

FIG. 2 a is an explanatory diagram representing the configuration of theaforementioned image display device wherein the optical path is unfoldedin the horizontal direction, while FIG. 2 b is a explanatory diagramrepresenting the configuration of the aforementioned image displaydevice wherein the optical path is unfolded in the vertical direction;

FIG. 3 is an explanatory diagram showing the diffusion characteristicsof the diffusing means of the aforementioned image display device;

FIG. 4 is an explanatory diagram showing an enlarged view of the displayregion when the diffusing direction of the diffusing means extends alongthe long-side direction of the display region of the display element;

FIG. 5 is an explanatory diagram giving an enlarged view of the displayregion when the diffusing direction of the diffusing means inclines withthe long-side direction of the display region of the display element;

FIG. 6 is an explanatory diagram showing an enlarged view of a displayregion of the display element equipped with color filters;

FIG. 7 is an explanatory diagram showing an enlarged view of the displayregion of another display element equipped with color filters;

FIG. 8 a is a plan view showing the schematic configuration of an HMDequipped with an image display device relating to another embodiment ofthe present invention, while FIG. 8 b is a side view of the HMD and FIG.8 c is a front view of the same;

FIG. 9 a is a plan view showing another HMD configuration, while FIG. 9b is a side view of the HMD and FIG. 9 c is a front view of the same;

FIG. 10 is a cross sectional view representing the schematicconfiguration of the image display device;

FIG. 11 is an explanatory diagram showing the optical path of the imagedisplay device unfolded optically in one direction;

FIG. 12 is an explanatory diagram showing the spectral intensitycharacteristics of the light source of the image display device;

FIG. 13 is an explanatory diagram showing the wavelength dependency ofthe diffraction efficiency in the hologram optical element of the imagedisplay device;

FIG. 14 is an explanatory diagram showing the optical path of the imagedisplay device relating to still another embodiment of the presentinvention, unfolded optically in one direction;

FIG. 15 is an explanatory diagram showing the relationship between theposition of the optical pupil in the X-axis direction and the intensityof light;

FIG. 16 is a cross sectional view showing the schematic configuration ofthe image display device relating to a further embodiment of the presentinvention;

FIG. 17 a is a cross sectional view showing an example of theconfiguration of the unidirectional diffusing plate applicable to theconventional image display device, while FIG. 17 b is an explanatorydiagram showing the shade of the unidirectional diffusing plate beingsuperimposed on the observed image; and

FIG. 18 is an explanatory diagram showing an enlarged view of thedisplay region of the display element of the image display device.

DESCRIPTION OF EMBODIMENTS Embodiment 1

The following describes an embodiment of the present invention withreference to the drawings.

(Image Display Device)

FIGS. 2 a and 2 b are explanatory diagrams representing the conceptualconfiguration of the image display device 1 of the present inventionwherein the optical path is unfolded. FIG. 2 a demonstrates thestructure with the optical path in the horizontal direction unfolded,and FIG. 2 b shows the same with the optical path in the perpendiculardirection unfolded. This image display device 1 includes a light source2, diffusing means 3, display element 4, and lens 5.

The light source 2 is composed of LEDs (devices for three colors areformed in one integral element) that emit each of three colors, R (red),G (green) and B (blue), for example, and a point light source is formed.The LEDs for R, G, and B are arranged along the horizontal direction asviewed from the observer. In the present embodiment, the display element4 to be described later is not equipped with a color filter. Thus, thelight source 2 sequentially emits each of the R, G, and B on atime-shared basis.

The diffusing means 3 has a diffusing surface 3 a wherein recesses andprotrusions are formed in one direction with plural pitches of about 1through 20 μm, for example, and forms a unidirectional diffusing plateto diffuse light traveling from the light source 2 in the one directionby the diffusion surface 3 a and emit the resulting light. In thepresent embodiment, the diffusing means 3 is arranged close to thedisplay element 4 to ensure that light will be diffused in thehorizontal direction as viewed from the observer. The details of thediffusing means 3 will be described later.

The display element 4 includes a plurality of pixels A (FIG. 4) and alight-shielding section BM (FIG. 4) formed around these pixels A. Thedisplay element 4 is an optical modulation element that modulates lighttraveling from the diffusing means 3 with the pixels A to display animage. In the present embodiment, the display element 4 is made of atransmission type LCD devoid of R, G, and B color filters. The displayelement 4 devoid of color filters provides a high transmittance andallows display of a bright image. The display element 4 is arranged suchthat the long-side direction of the display region extends along thehorizontal direction.

The lens 5 has a positive optical power, for example, and guides lightof an image traveling from the display element 4 to the optical pupil Eto constitute an ocular lens system (optical system for observation)which makes an observer observe a virtual image of the image at theposition of the optical pupil E. To be more specific, the lens 5 formsan optical pupil E so that an observer can observe an image displayed bydisplay element 4 as a virtual image.

In the aforementioned configuration, light emitted from the light source2 which is almost a point light source is diffused in one direction (inthe horizontal direction) by the diffusing means 3. After that, thelight is modulated by the display element 4, and is emitted as imagelight therefrom. This image light is led to the optical pupil E throughthe lens 5. It allows an observer to observe a virtual image of theimage displayed by the display element 4, at the position of the opticalpupil E. In this case, synchronously with the emission of the light ofR, G, and B colors by the light source 2, each of the pixels of thedisplay element 4 is driven, which allows the observer to observe acolor image.

Use of the point light source as the light source 2 eliminates the needof increasing the degree of diffusion to reduce the luminancenonuniformity of the light source, as compared to the case of a surfacelight source, and provides a bright image to be observed. Further, sucha large current as that for driving the surface light source need not besupplied to the light source 2. This signifies saving of powerconsumption.

(Diffusing Means)

The following describes the details of the diffusing means 3. As shownin FIGS. 2 a and 2 b, the surface on the side opposite the diffusingsurface 3 a of the diffusing means 3 is a non-diffusing surface 3 b. Inthe present embodiment, the diffusing means 3 is arranged such that thediffusing surface 3 a faces the display element 4. To put it anotherway, the non-diffusing surface 3 b of the diffusing means 3 forms alight-incident surface where light from the light source 2 enters, andthe diffusing surface 3 a forms a light-outgoing surface. The followingdescribes the reason for this arrangement of the diffusing means 3.

For example, under the condition that the diffusing means 3 is arrangedwith the diffusing surface 3 a formed of recesses and protrusions facingthe light source 2, when light emitted from light source 2 and reflectedby non-diffusing surface 3 b of the diffusing means 3 forms backsidereflection light and the backside reelection light reaches the diffusingsurface 3 a again, the light can be totally reflected. In this case, theluminance nonuniformity will be increased because the light totallyreflected by the diffusing surface 3 a does not enter the displayelement 4. However, if the diffusing means 3 is arranged such that thediffusing surface 3 a faces the display element 4, it is possible toreduce the amount of the light returned by the backside reflected light(the light reflected by the diffusing surface 3 a and traveling towardthe non-diffusing surface 3 b), with the result that the aforementionedluminance nonuniformity can be reduced.

Incidentally, FIG. 3 is an explanatory diagram showing the diffusioncharacteristics of the diffusing means 3. Assume that θh denotes thedegree of diffusion of the diffusing means 3 in the horizontal directionas viewed from the observer (also referred to as “in the lateraldirection”), and θv indicates the degree of diffusion of the diffusingmeans 3 in the vertical direction as viewed from the observer (alsoreferred to as “in the longitudinal direction”). In this case, thedegree of diffusion of the diffusing means 3 can be expressed as θh=40degrees and θv=0.2 degree in terms of intensity at half maximum. Namely,the diffusing means 3 allows the incident light to be diffused within 40degrees in the lateral direction and 0.2 degree in the longitudinaldirection, as viewed from the observer, by the diffusing surface 3 a(FIGS. 2 a and 2 b) made of recesses and protrusions.

As described above, the diffusing means 3 hardly allows the incidentlight to be diffused in the longitudinal direction as viewed from theobserver. Thus, the diffusing surface 3 a has no ruggedness in thelongitudinal direction as shown in FIG. 2 h, and has a structure ofrecesses and protrusions in the lateral direction wherein randomruggedness is formed, as shown in FIG. 2 a. When the diffusing means 3allows the incident light to be diffused in one direction (in thelateral direction), the size of the optical pupil E will be 10 mm, forexample, in the lateral direction and 1.5 mm, for example, in thelongitudinal direction. The light is converged in the longitudinaldirection wherein the optical pupil E is smaller, and this ensures abright image (virtual image) to be observed by the observer. In themeantime, in the lateral direction wherein the optical pupil E isgreater, easier observation of the image is ensured for the observer.

In the present embodiment, θh=40 degrees and θv=0.2 degree, as describedabove. The degree of diffusion of the diffusing means 3 in the diffusingdirection is ten or more times larger than that in the directionperpendicular to the diffusing direction. Thus, the degree of diffusionin the perpendicular direction to the diffusing direction is maderelatively smaller than that in the diffusing direction of so that theincident light is hardly diffused in the perpendicular direction. Thisarrangement provides the observer with a bright image.

(Reduction in Luminance Nonuniformity and Color Nonuniformity)

The following describes the conditions for reducing the luminancenonuniformity and color nonuniformity of an image to be observed, causedby the shade of the recesses and protrusions on the diffusing surface 3a of the diffusing means 3.

The diffusing means 3 formed of a unidirectional diffusing plate iscreated, for example, by forming a structure of recesses and protrusionsalong one axis with a plurality of pitches of about 1 to 20 μm on atransparent resin substrate having a thickness of about 0.1 mm. Here thediffusing means 3 with θh=40 degrees, for example, has a structure ofrecesses and protrusions having a maximum pitch of about 15 μm. If thestructure of recesses and protrusions is observed by an ocular opticalsystem with a focal length of 20 mm, for example, the viewing angle isabout three minutes. This is greater than the viewing angle of oneminute equivalent to the resolution of the human eye. Thus, if theintensity of the outgoing light is reduced because of the totalreflection at the portion having a greater incident angle in theinclined portion of the recesses and protrusions, the reduction in theintensity is identified by the observer as a streak of shade. Further,the diffusing means 3 having a small degree of diffusion of θh=30 to 10degrees is provided with a structure of recesses and protrusions with apitch of about 20 μm. This ensures easier identification of the shadesby the observer. Luminance nonuniformity and color nonuniformity are theresult of superimposition of these shades on the image to be observed.

Thus, in the present embodiment, the following steps are taken to makean observer observe a high-quality image in which luminancenonuniformity and color nonuniformity are reduced, while the degree ofdiffusion of the diffusing means 3 is reduced and the image makesbrighten.

FIG. 1 is an explanatory diagram for schematically illustrating variousparameters of the image display device 1 as the present embodimentAssume that Hw (mm) indicates the aperture width of the pixels A of thedisplay element 4 in the diffusing direction of the diffusing means 3,Kw (mm) denotes the maximum pitch of the recesses and protrusions in thediffusing direction of the diffusing means 3, f (mm) shows the focallength of the lens 5, and 8 (mm) represents the distance from thedisplay surface of the display element 4 to the diffusing surface 3 a ofthe diffusing means 3. In this case, the image display device 1 meetsthe following conditional expression (1):

Hw/f≦tan 1′≦Kw/(f+δ)  (1)

The display surface of the display element 4 is located close to theimage surface of the lens 5 (focal length f). This arrangement forms theoptical pupil E that supplies the observer with a virtual image.

In this case, Hw/f≦tan 1′ defines the conditions for ensuring that theshade of the recesses and protrusions of the diffusing means 3 is noteasily identified by the observer. The expression: tan 1′≦Kw/(f+δ)defines the conditions for supplying the observer with a bright image byincreasing the pitch of the recesses and protrusions of the diffusingsurface 3 a of the diffusing means 3, namely, by reducing the degree ofdiffusion. The following describes the details.

The resolution of the human eye is said to be equivalent to one minutein terms of viewing angle. To be more specific, when a human beingobserves an object at a viewing angle of 1 minute or more, the objectcan be identified as an image. In the meantime, Kw/(f+δ) is almostequivalent to the tangent of the viewing angle (viewing angle α inFIG. 1) of the maximum pitch of the recesses and protrusions in thediffusing direction of the diffusing means 3. Further, Hw/f is almostequivalent to the tangent of the viewing angle (viewing angle β inFIG. 1) of one pixel of the display element 4.

If tan 1′≦tan α holds, namely, tan 1′≦Kw/(f+δ) holds, the pitches of therecesses and protrusions can be enlarged, and the degree of diffusion ofthe diffusing means 3 can be reduced, so that the observer is providedwith a bright image. On the other hand, however, the shade of therecesses and protrusions of the diffusing surface 3 a can be identifiedas a virtual image by the observer. This shade produces the luminancenonuniformity and color nonuniformity of the displayed image, as aresult.

However, when the tan β≦tan 1′, namely, Hwf≦/tan 1′ is satisfied, theaperture width of the pixels A of the display element 4 can be reduced,and a large part of the shade of the recesses and protrusions of thediffusing surface 3 a (the shade of the recesses and protrusions at theviewing angle greater than the viewing angle corresponding to theresolution of the human eye) can be superimposed on the light-shieldingsection BM of the display element 4. Further, even if the shade has beensuperimposed on the pixels A of the display element 4, since theaperture width Hw of the pixels A is smaller than that corresponding tothe resolution of the human eye, the observer cannot easily identify theimage through the pixel.

Thus, when the conditional expression (1) has been met, the observer canbe provided with the high-quality image with reduced luminancenonuniformity and color nonuniformity, whereas the degree of diffusionof the diffusing means 3 is reduced and a bright image is displayed.

When Hp is assumed to indicate the pitch of the pixels of the displayelement 4 in the diffusing direction of the diffusing means 3, theaperture efficiency Hw/Hp of the pixel of the display element 4 in thediffusing direction of the diffusing means 3 preferably meets thefollowing conditional expression (1A)

0.3≦Hw/Hp≦0.8  (1A)

The aperture efficiency Hw/Hp indicates the percentage of thetransmission region (aperture) in the diffusing direction of thediffusing means 3 of the display element 4. If the aperture efficiencyHw/Hp is greater, the aperture width Hw is increased and thelight-shielding section BM is reduced. If the aperture efficiency Hw/Hpis smaller, the aperture width Hw is reduced and the light-shieldingsection BM is increased. If the upper limit of the conditionalexpression (1A) has been exceeded, the shade of recesses and protrusionsof the diffusing surface 3 a will be likely to provide a moreconspicuous influence. If the lower limit of the conditional expression(1A) cannot be reached, the volume of transmitting light will bereduced, with the result that the image will appear darker for theobserver. The conditional expression (1A) provides a preferable valuedetermined with consideration given to the influence of the shaderesulting from the recesses and protrusions of the diffusing surface 3 aand the brightness of the image.

FIG. 4 is an explanatory diagram showing an enlarged view of the displayregion of the display element 4. It will be apparent that, when theconditional expression (1) is met, the shade P of the recesses andprotrusions of the diffusing means 3 is hidden by the light-shieldingsection BM of the display element 4 and cannot be seen easily. Thisdiagram clearly indicates the shade P of the recesses and protrusionsfor ease of explanation. As discussed above, the viewing angle of thepixels A is smaller than the viewing angle of one minute, whichcorresponds to the resolution of the human eye. This arrangementincreases difficulties in the identification by the observer. When theimage is actually observed, the image will be slightly out of focus dueto the presence of the diffusing means 3. Thus, an average is taken forthe shade P of the recesses and protrusions and bright portion (displayof pixels A), with the result that observation difficulties will beincreased.

It should be noted that the human eye cannot resolve the fine recessesand protrusions (at a pitch of 5 μm or less with a focal length of thelens 5 of 20 mm) of the diffusing means 3 which provides a viewing anglesmaller than viewing angle of one minute corresponding to the resolutionof the human eye. Thus, the shade of the recesses and protrusions havingbeen reduced to an averaged level by the eyes cannot be easily observed.

In the present embodiment, the diffusing means 3 uses a unidirectionaldiffusing plate that diffuses the incident light within 0.5 degree inthe direction perpendicular to the diffusing direction (wherein thisunidirectional diffusing plate hardly diffuses the incident light in thevertical direction). The observer can also be supplied with a brighthigh-quality image by using, for example, a unidirectional diffusingplate that diffuses the incident light within 3 degrees in theperpendicular direction.

(Aperture width Vw)

As shown in FIG. 4, when Vw (mm) indicates the aperture width of thepixels A of the display element 4 in the direction perpendicular to thediffusing direction of the diffusing means 3, the image display device 1of the present embodiment meets the following conditional expression(2):

tan 1′≦Vw/f  (2)

Vw/f approximately corresponds to the tangent of the viewing angle ofthe pixels A of the display element 4 in the direction perpendicular tothe diffusing direction of the diffusing means 3. The diffusing means 3hardly diffuses the light in the direction perpendicular to thediffusing direction. Thus, by providing the expression: tan1′≦Vw/f andenlarging the aperture width of the pixels A in the directionperpendicular to the diffusing direction to form the aperture widthgreater than that corresponding to the human eye, the observer can beprovided with a bright image.

(Distance δ)

Distance δ from the display surface of the display element 4 to thediffusing surface 3 a of the diffusing means 3 is preferably set toensure that the difference in visibility between the virtual imageformed by the lens 5 of the diffusing means 3 and the virtual image ofthe image displayed on the display element 4 will be ten or morediopters. In the embodiment, it is set to ensure that the difference invisibility of the virtual images will be three diopters, for example.The following describes the reason for this arrangement.

In the first place, the following briefly describes the aforementioneddiopter. Diopter is expressed by the reciprocal of the focal length of alens represented in meters (1/m). It is commonly used as a unitrepresenting the visibility, namely, the power of the lens (refractivepower). Thus, in the present embodiment, when the virtual image of theimage displayed on the display element 4 is located, for example, onemeter in front of the observer's eyes (if the virtual image of the imageis located at −1d (diopter)), the virtual image through the lens 5 ofthe diffusing means 3 is located −4d (25 cm in front of the eyes) or +2d(infinite distance in front of the eyes (at 0d it becomes an infinitedistance in front of the eyes).

Ten diopters indicate the maximum adjustable range of the focus of thehuman eye. If the difference in the visibility between the virtual imagethrough the lens 5 of the diffusing means 3 and virtual image of thedisplay image of the display element 4 does not exceed ten diopters, notonly the virtual image of the display image, but also the virtual imageof the diffusing means 3 (including the shade of the recesses andprotrusions) are kept within the range observable to the human eye. Inthe present invention, however, when the aforementioned conditionalexpression (1) is met, there will be difficulties in the identificationof the shade of the recesses and protrusions of the diffusing means 3.This ensures a high-quality image to be viewed by the observer even whenthe distance δ is set to ensure that the difference in the visibilitybetween both virtual images does not exceed ten diopters, and thediffusing means 3 is placed close to the display element 4. Thus, theefficiency in the use of light from the diffusing means 3 can beenhanced and a brighter image can be viewed by the observer, when thediffusing means 3 is placed close to the display element 4 and thedevice is designed in a compact and lightweight structure.

(Another Layout of the Diffusing Means)

The above description has been made with reference to the examplewherein the diffusing means 3 is arranged to ensure that light will bediffused in the long-side direction of the rectangular display element 4(in the direction of the pixels A arrayed along the long side). However,the diffusing means 3 can be arranged (by rotation) so that thediffusing direction will intersects with the long-side direction of therectangular display region of the display element 4, for example.

When the diffusing means 3 allows the diffusing surface 3 a of therecesses and protrusions to diffuse the incident light in one direction,the shade of the recesses and protrusions of the diffusing means 3 isgenerated as a streak in the direction perpendicular to the diffusingdirection. In this case, for example, if the diffusing direction of thediffusing means 3 is matched with the long-side direction of the displayregion of the display element 4, the shade P of the recesses andprotrusions of the diffusing means 3 will be superimposed on a pluralityof pixels A located in the direction perpendicular to the diffusingdirection, namely, in the short-side direction of the display region, asshown in FIG. 4. Thus, the shade will be split by the light-shieldingsection BM of pixels A only in the short-side direction.

FIG. 5 is an explanatory diagram giving an enlarged view of the displayregion of the display element 4 when the diffusing direction of thediffusing means 3 inclines at five degrees inclined with respect to thelong-side direction of the display region of the display element 4. Evenif the diffusing direction of the diffusing means 3 crosses thelong-side direction of the display region of the display element 4, theshade P of the recesses and protrusions of the diffusing means 3 issuperimposed on a plurality of pixels A located in the directionperpendicular to the diffusing direction. In this case, the shade issplit by the light-shielding section BM around pixels A in bothdirections of the long and short sides of the display region, namely, inthe inclined direction. This will increase the difficulty of the shade Pof the diffusing means 3 being identified by the observer.

(Another Example Showing the Configuration of the Display Element)

The above description has been made with reference to an example ofusing the display element 4 not provided with color filters. It goeswithout saying that a display element provided with a color filter canbe used. In this case, the light source 2 is only required to causesimultaneous emission of light from each of the R, G, and B LEDs.

For example, FIG. 6 is an explanatory diagram showing an enlarged viewof a display region of the display element 4 a equipped with colorfilters. In the display element 4 a, filters are arranged in the orderof R, G, and B in the long-side direction of the rectangular displayregion, and filters of one and the same color are located in theshort-side direction of the rectangular display region. FIG. 7 is anexplanatory diagram showing an enlarged view of the display region ofanother display element 4 b equipped with color filters. In the displayelement 4 b, filters are arranged in the order of R, G, and B in thelong-side direction of the rectangular display region, and filters arealso located in the short-side direction of the rectangular displayregion. If the diffusing direction of the diffusing means 3 is made toagree or intersect with the long-side direction of the display region,use of the display elements 4 a and 4 b equipped with color filters canincrease the difficulty for the observer to view the shade of therecesses and protrusions of the diffusing means 3, similarly to thecases wherein the display element 4 is used.

In the display element 4 b of FIG. 7 in particular, even if astreak-formed shade is formed in the direction perpendicular to thediffusing direction under the condition that, for example, the diffusingdirection is the same as the direction of the long side of the displayregion, the specific color alone is not weakened (luminance is notreduced) because of this shade. Accordingly, this arrangement providesthe observer with the high-quality image with further reduction in theluminance nonuniformity and color nonuniformity.

In the display elements 4 a and 4 b, assume that Dw (mm) indicates thepitch of the pixels A of the same color in the diffusing direction. ThenDw≧tan α, namely, Dw≧Kw/(f+δ) can hold, if the conditional expression(1) is met. To be more specific, in the display elements 4 a and 4 b,when the aperture width Hw of the pixels A of each of the R, G, and B issmaller, it is possible to increase the difficulty for the observer toview the shade of the recesses and protrusions of the diffusing means 3,similarly to the case wherein the display element 4 is used. Thiseliminates the need of making the value of Dw smaller than tan α.

Embodiment 2

The following describes another embodiment of the present invention withreference to the drawings.

(HMD Structure)

FIG. 8 a is a plan view showing the schematic configuration of an HMD ofthe present embodiment, while FIG. 8 b is a side view of the HMD, andFIG. 8 c is a front view of the same. The HMD includes an image displaydevice 11 and a supporting means 12 supporting the same. The generalstructure of the HMD is so designed that one (left one, for example) ofthe lenses has been removed from the general spectacles.

The image display device 11 allows the observer to view the image of theexternal world in the see-though mode, and displays the image to providethe observer with that image as a virtual image. This image displaydevice 11 corresponds to the image display device 1 of the firstembodiment. In the image display device 11 of FIG. 8 c, the portioncorresponding to the right-eye lens of the spectacles is made up of anocular prism 32 and deflecting prism 33 which are bonded together, aswill be described later. The detailed structure of the image displaydevice 11 will also be described later.

The supporting means 12 is a supporting member which supports the imagedisplay device 11 in front of an eye of the observer (e.g., in front ofthe right eye), and includes a bridge 13, frames 14, temples 15, nosepads 16, cable 17, and control means for transmittance of external light18. A pair of frames 14, temples 15, and nose pads 16 is provided on theright and left. When they are to be distinguished between the tight-handside and left-hand side, they will be described as a right frame 14R,left frame 14L, right temple 15R, left temple 15L, right nose pad 16R,and left right nose pad 16L.

One end of the image display device 111 s supported by the bridge 13.The bridge 13 also supports the left frame 14L, nose pads 16 and controlmeans for transmittance of external light 18 in addition to the imagedisplay device 11. The left frame 14L rotatably supports the left temple15L. In the meantime, the other end of the image display device 11 issupported by the right frame 14R. In the right frame 14R, the endopposite the supporting side of the image display device 11 rotatablysupports the right temple 15R. The cable 17 is used to supply the imagedisplay device 11 with an external signal (e.g., image signal andcontrol signal) and electric power, and is provided along the rightframe 14R and right temple 15R. The control means for transmittance ofexternal light 18 is provided on the bridge 13 to control thetransmittance of the external light (light of an external image), and islocated in front of the image display device 11 (opposite the observer).

When the observer uses the HMD, the right temple 15R and left temple 15Lare brought into contact with the right side and left side of the headof the observer and the nose pads 16 are applied to the nose of theobserver. The observer wears the HMD on the head in the manner ofwearing a pair of spectacles. If an image is displayed by the imagedisplay device 11 under this condition, the observer is allowed to viewthe image of the image display device 11 as a virtual image. At the sametime, the observer can view the external image through the image displaydevice 11 in a see-through mode.

In this case, if the transmittance of external light is set to the levelof 50 percent or less, for example, in the control means fortransmittance of external light 18, easier viewing of the image of theimage display device 11 by the observer is ensured. Conversely, if thetransmittance of external light is set to the level of 50 percent ormore, the observer is allowed to have easier viewing of an externalimage. Accordingly, the transmittance of external light in the controlmeans for transmittance of external light 18 should be set to anappropriate level in such a way as to ensure easy observation of theimage of the image display device 11 and external image.

As described above, when the image display device 11 is supported by thesupporting means 12, the observer is allowed to enjoy hands-free viewingof the image supplied by the image display device 11.

It should be noted that the HMD is not restricted to the one having onlyone image display device 11. For example, FIG. 9 a is a plan viewshowing another HMD configuration, while FIG. 9 b is a side view of theHMD and FIG. 9 c is a front view of the same. As described above, theHMD can be provided with two image display devices 11, 11. In this case,the image display device 11 placed in front of the left eye is supportedbetween the bridge 13 and left frame 14L. Further, the two image displaydevices 11,11 are connected with each other by the cable 17, andexternal signals and others are supplied to two image display devices11, 11 through the cable 17.

(Details of Image Display Device)

The following describes the details of the image display device 11.

FIG. 10 is a cross sectional view representing the schematicconfiguration of the image display device 11. FIG. 11 is an explanatorydiagram showing the optical path of the image display device 11 unfoldedoptically in one direction. The image display device 11 includes a lightsource 21, light-converging lens 22, diffusing means 23, display element24, and ocular optical system 31. As shown in FIG. 10, the light source21, light-converging lens 22, diffusing means 23, and display element 24are housed in a casing 20. Part of the ocular optical system 31 (part ofthe ocular prism 32 to be described later) is located inside the casing20. The image display device 11 meets both the conditional expressions(1) and (2).

For the sake of expediency, the following defines the directions: Assumethat the optical axis denotes the axis optically connecting between thecenter of the display region of the display element 24 and the center ofthe optical pupil E formed by the ocular optical system 31. Also assumethat the Z-axis direction indicates the direction of the optical axisobtained by unfolding the optical path from the light source 21 to theoptical pupil E, and the X-axis direction represents the directionperpendicular to the optical-axis incident surface of a hologram opticalelement 34 (to be described later) of the ocular optical system 31. TheY-axis direction is the direction perpendicular to the ZX plane. Theoptical-axis incident surface of the hologram optical element 34indicates the plane (i.e., YZ plane) including the optical axis of theincident light to the hologram optical element 34 and the optical axisof the reflected light. The above-mentioned optical-axis incidentsurface may be called the incident surface in the following description.

The light source 21 is composed of a LED wherein elements for R, G, andB are integrated into one body, including three light-emitting chipsemitting light of three colors of R, G, and B as light-emitting sections21R, 21G, and 21B (FIG. 11). LEDs are less costly and designed in acompact structure. Further, the width of the emitted light wavelength issmaller, as will be described later. Thus, the LED provides high colorpurity. Accordingly, when the light source 21 is composed of the LED ofR, G, and B, the image display device 11 with reduced costs and compactsize can be achieved, and the observer is provided with an image of highcolor purity.

The light-emitting sections 21R, 21G, and 21B each have a size of about0.3 mm square, and are arranged with a pitch of 0.5 mm along the X-axisdirection. When providing the light-emitting sections 21R, 21G and 21Barranged in the X-axis direction as the diffusing direction of thediffusing means 23 (to be described later), colors of R, G, and B aremixed in the above-mentioned diffusing direction with the diffusingmeans 23. This reduces the intensity nonuniformity of each color on theoptical pupil E, and therefore, reduces the color nonuniformity.

The light-emitting sections 21R, 21G, and 21B need not be arrangedperfectly in a line in the X-axis direction. For example, part of thelight-emitting sections 21R, 21G, and 21B may be displaced by 0.5 mm inthe Y-axis direction. In this case, the light source 21 can be arrangedin such a way that a greater number of light-emitting sections will bearranged in a straight line in the X-axis direction.

The light-converging lens 22 converges light traveling from the lightsource 21 in the direction (Y-axis direction) perpendicular to thediffusing direction of the diffusing means 23, and guides the light tothe diffusing means 23. The light-converging lens 22 includes acylindrical lens, for example. Use of the light-converging lens 22allows the optical system for illuminating the display element 24 to bedesigned in a compact and lightweight structure. The optical power ofthe light-converging lens 22 in the direction (X-axis direction)parallel with the diffusing direction of the diffusing means 23 is zeroor negative. This ensures that the light from the light source 21 is notconverged in the X-axis direction. This allows the observer to besupplied with a high-quality image, without the luminance nonuniformity(intensity nonuniformity) being increased in the X-axis direction.

The light-converging lens 22 is arranged such that, after thelight-converging lens converges light from the light source 21, thelight diffused by diffusing means 23 effectively forms the optical pupilE. Further, the light-converging lens 22 and hologram optical element 34(to be described later) are arranged such that the light source 21 andoptical pupil E are conjugate with respect to the direction (Y-axisdirection) perpendicular to the diffusing direction of the diffusingmeans 23. In the Y-axis direction of the optical pupil E, thelight-emitting area (for example, 0.3 mm square) is formed slightlylarger than the pupil formed at the image magnification in a conjugaterelationship, by the diffusion of one degree caused in the diffusingmeans 23 and the diffusion of about two degrees caused in the displayelement 24. As a result, the optical pupil E has a size of 8 mm in theX-axis direction and 2 mm in the Y-axis direction in this embodiment.

As described above, in one direction (the X-axis direction), the opticalpupil E has a size of 8 mm, which is greater than the human pupil (about3 mm). This ensures easier viewing of the image by the observer. In themeantime, in the other direction (the Y-axis direction), the opticalpupil E has a size of 2 mm, which is smaller than the human pupil. Thispermits the light from the light source 21 to be effectively convergedinto the optical pupil E in the above-mentioned direction, and theobserver is provided with a bright image. In the present embodiment, theoptical pupil is made smaller than the human pupil to provide a brighterimage. If the optical pupil is small in one direction, a bright imagecan be provided, even if the optical pupil can be made larger than thehuman pupil.

In the Y-axis direction perpendicular to the diffusing direction of thediffusing means 23, the light source 21 and optical pupil E arepositioned at conjugate positions with each other. This allows the sizeof the optical pupil E to be further reduced in the Y-axis direction,and permits the efficiency of using the light from the light source 21to be increased in the Y-axis direction, whereby the observer isprovided with a bright image. In the X-axis direction, the incidentlight is diffused widely by the diffusing means 23, and the conjugaterelationship between the light source 21 and optical pupil E is notestablished. However, the light from the light source 21 can be used athigher efficiency by the conjugate arrangement, with the result that abright image is displayed.

The diffusing means 23 is a unidirectional diffusing plate including adiffusing surface 23 a in which recesses and protrusions are formed witha plurality of pitches in one direction, wherein light from the lightsource 21 is diffused and emitted in one direction by the diffusingsurface 23 a. The diffusing means 23 corresponds to the diffusing means3 of the first embodiment. In the present embodiment, the diffusingmeans 23 is arranged close to the display element 24 such that itsdiffusing direction agrees with the horizontal direction (the X-axisdirection) of the observer and that a difference in visibility between avirtual image formed by the ocular optical system 31 of the diffusingmeans 23 and a virtual image of an image displayed in display element isfive diopters. The degree of diffusion in the X-axis direction of thediffusing means 23 is 30 degrees, for example. The opposing surface tothe diffusing surface 23 a in the diffusing means 23 forms anon-diffusing surface, similarly to the case of the first embodiment.The diffusing means 23 is arranged so that the diffusing surface 23 awill face the display element 24.

The display element 24 is used to modulate the light emitted from thelight source 21 through the diffusing means 23, in response to the imagedata, and to display an image. The display element 24 is provided withR-, G-, and B-color filters. The display element 24 has a matrix ofpixels forming a region to allow transmission of light, and is composedof a transmission type LCD with a light-shielding section formed aroundeach of the pixels. The display element 24 is arranged such that thelong-side direction of the rectangular display region corresponds to theX-axis direction and the short-side direction corresponds to the Y-axisdirection.

The ocular optical system 31 is an optical system for observationwherein the image light from the display element 24 i.e., the lightcorresponding to the image displayed in the display element 24 is led tothe optical pupil E, so that the virtual image of the above-mentionedimage is viewed by the observer at the position of the optical pupil E.The ocular optical system 31 includes an ocular prism 32 (firsttransparent substrate), deflecting prism 33 (second transparentsubstrate) and hologram optical element 34.

The ocular prism 32 totally reflects the image light from the displayelement 24 inside, and guides this light to the optical pupil E throughthe hologram optical element 34. At the same time, the ocular prism 32allows transmission of external light and guides it to the optical pupilE. Together with the deflecting prism 33, the ocular prism 32 iscomposed of acrylic resin, for example. This ocular prism 32 is formedin such a way that the bottom end of the parallel flat plate is shapedlike a wedge, and the top end is made thick. The ocular prism 32 isbonded with the deflecting prism 33 by adhesive in such a way as tosandwich the hologram optical element 34 arranged on the bottom end.

The deflecting prism 33 is formed of an approximately U-shaped parallelflat plate as in plan view (FIG. 8 c). When bonded with the bottom endand surfaces on both side ends (end faces on the right and left) of theocular prism 32, the deflecting prism 33 is integrated with the ocularprism 32 and is formed into an approximately parallel plate. When thisdeflecting prism 33 is bonded with the ocular prism 32, the observer isallowed to prevent distortion from occurring to the external image to beobserved, through the ocular optical system 31.

To be more specific, for example, if the deflecting prism 33 is notbonded with the ocular prism 32, the external light is refracted whenpassing through the wedge-like bottom end of the ocular prism 32. Thiscauses distortion to occur to the external image viewed through theocular prism 32. However, when the deflecting prism 33 is bonded withthe ocular prism 32 to form an integral approximately parallel plate,the refraction of the external light passing through the bottom end ofthe ocular prism 32 is offset by the deflecting prism 33. This preventsoccurrence of the distortion to the external image to be observed in asee-through mode.

Each of the surfaces of the ocular prism 32 and deflecting prism 33 (twoopposing surfaces of the parallel plate) can be flat or curved. If eachof the surfaces of the ocular prism 32 and deflecting prism 33 iscurved, the ocular optical system 31 can be provided with a function ofa corrective lens.

The hologram optical element 34 is a hologram optical element of avolume phase type and a reflection type that diffracts and reflects theimage light (light having wavelengths corresponding to three primarycolors) emitted from the display element 24 and leads the image light tothe optical pupil, whereby the image displayed by the display element 24is enlarged and is led to the pupils of the observer as a virtual image.The hologram optical element 34 has an axi-asymmetric positive opticalpower, namely, the same function as that of a mirror with an asphericconcave surface having a positive power. This increases the degree offreedom in the layout of the optical members constituting the product tobe arranged, and allows the product to be designed in a compactstructure. At the same time, an effectively aberration-corrected imagecan be supplied to the observer.

(Operation of the Image Display Device)

The following describes the operation of the image display device 11.Light emitted from light source 21 is converged by the light-converginglens 22, then, is diffused by the diffusing means 23 to enter thedisplay element 24 as uniform illumination light with excellent R, G,and B color mixture. The light having entered the display element 24 ismodulated in each pixel according to the image data, and is outputted asimage light. Thus, a color image is displayed on the display element 24.

The image light from the display element 24 enters the ocular prism 32of the ocular optical system 31 from the top end face (32 a). Afterhaving been totally reflected a plurality of times by two opposingsurfaces 32 b and 32 c, this image light enters the hologram opticalelement 34. The light having entered the hologram optical element 34 isreflected at that position to reach the optical pupil E. This allows theobserver to view an enlarged virtual image of the image displayed on thedisplay element 24, at the position of the optical pupil E.

In the meantime, the ocular prism 32 and deflecting prism 33 almostcompletely permit passage of the external light, and this allows theobserver to view the external image. Thus, the virtual image of theimage displayed on the display element 24 is observed in the form partlysuperimposed with the external image.

As described above, the image display device 11 permits the image lightemitted from the display element 24 to be totally reflected inside theocular prism 32 so that the light is led to the hologram optical element34. This structure allows the thickness of the ocular prism 32 anddeflecting prism 33 to be about 3 mm, similarly to the case of thecommonly used lenses of the spectacles. Thus, the image display device11 can be designed in a compact and lightweight structure. Further, useof the ocular prism 32 capable of totally reflecting the image lightfrom the display element 24 ensures a high transmittance of externallight, with the result that the observer is provided with a brightexternal image.

The hologram optical element 34 serves as a combiner that leads theimage light from the display element 24 and external light to the pupilsof the observer simultaneously. Thus, the observer can simultaneouslyview the image provided by the display element 24 and external image,through the hologram optical element 34.

In the present embodiment, the difference in visibility between thevirtual image provided by the ocular optical system 31 of the diffusingmeans 23 and the virtual image displayed on the display element 24 isfive diopters. Not only the virtual image of the display image but alsothe virtual image provided by the ocular optical system 31 of thediffusing means 23 are kept within the range visible to the human eye(without exceeding ten diopters). However, when the conditionalexpressions shown in the first embodiment is met, it becomes difficultto visually identify the shade of the recesses and protrusions of thediffusing means 23. Thus, even when the diffusing means 23 is placedclose to the display element 24, luminance nonuniformity and colornonuniformity occurring to the virtual image of the displayed image canbe reduced, with the result that a bright high-quality image (virtualimage) can be viewed by the observer.

Especially the image display device 11 that allows the external image tobe viewed in a see-through mode as in the present embodiment is requiredto ensure easy viewing of the display image (virtual image) superimposedon the external image as a bright image by the observer. Thus, meetingthe conditional expressions of the first embodiment is very effective,especially in the see-through mode image display device 11 and HMD.

(Characteristics of Light Source and Hologram Optical Element)

The following describes the characteristics of the light source 21 andhologram optical element 34.

FIG. 12 is an explanatory diagram showing the spectral intensitycharacteristics of the light source 21, namely, the relationship betweenthe wavelength of the outgoing light and light intensity. The lightsource 21, for example, emits light having three wavelength bands,462±12 nm, 525±17 nm and 635±11 nm at the peak wavelength of the lightintensity and wavelength width of the light intensity at half maximum.The light intensity plotted on the vertical axis of FIG. 12 is given inthe relative value wherein the maximum light intensity of light B isassumed as 100.

The peak wavelength of the light intensity is defined as the wavelengthat which the light intensity has reached the peak level. The wavelengthwidth of the light intensity at half maximum is defined as thewavelength width at which the light intensity is the half value of thepeak light intensity. The light intensity for R, G, and B of the lightsource 21 can be adjusted with consideration given to the diffractionefficiency of the hologram optical element 34 and light transmittance ofthe display element 24, whereby display in white color can beimplemented.

FIG. 13 is an explanatory diagram showing the wavelength dependency ofthe diffraction efficiency in the holographic optical element 34. Asillustrated, the hologram optical element 34, for example, ismanufactured to diffract (reflect) the light in three wavelength bands,465±5 nm (light B), 521±5 nm (light G), and 634±5 nm (light R) at thepeak wavelength of diffraction efficiency and the wavelength width ofthe diffraction efficiency at half maximum. Here the peak wavelength ofdiffraction efficiency is defined as the wavelength at which thediffraction efficiency reaches the peak level, and the wavelength widthof the diffraction efficiency at half maximum refers to the wavelengthwidth when the diffraction efficiency has reached the half value of thepeak diffraction efficiency. The diffraction efficiency of FIG. 13 isgiven in the relative value wherein the maximum diffraction efficiencyof light B is assumed as 100.

As described above, the hologram optical element 34 is designed todiffract only the light of a specific wavelength having a specificincident angle, and does not affect transmittance of the external light.This enables the observer to view the external image in the same way asusual, through the ocular prism 32, hologram optical element 34, anddeflecting prism 33.

In the hologram optical element 34 of volume phase type and reflectiontype, the diffraction efficiency is high and the half-value wavelengthwidth of the peak diffraction efficiency is small, as shown in FIG. 13.Accordingly, the image light from the display element 24 is diffractedand reflected by the hologram optical element 34, using such a hologramoptical element 34, and is led to the optical pupil E. This structuresupplies the observer with a bright image of high color purity. Further,the transmittance of the external light is also increased. Thus, theobserver is provided with a bright external image. To be more specific,the observer is supplied with an image of higher visibility superimposedon the bright external image.

Further, from the above-mentioned numerical relationship, the peakwavelength of the diffraction efficiency of the hologram optical element34 can be said to be approximately the same as the peak wavelength ofthe light intensity emitted from the light source 21. In this setting,the light with a wavelength and its peripheral wavelengths at which thelight intensity reaches the peak out of light emitted from light source21, is effectively diffracted by the hologram optical element 34. Thus,even when this light is superimposed on the external image, the observeris provided with a bright, easy-to-see image.

(Color Nonuniformity Reducing Effect)

In this embodiment, the optical pupil E is designed to have 8 mm in theX-axis direction and 2 mm in the Y-axis direction with the intensity athalf maximum, as described above. To be more specific, the optical pupilE is greater in size in the X-axis direction, namely, the directionperpendicular to the incident surface (YZ plane) of the hologram opticalelement 34 than in the Y-axis direction, namely, the direction parallelto the above-mentioned incident surface. When the optical pupil E is setto this size, the observer can be provided with a high-quality image ofreduced color nonuniformity, without much affected by the wavelengthcharacteristics (wavelength selectivity) of the hologram optical element34. The reason for that is described below.

The following describes the relationship between the incident angle inthe hologram optical element 34 and the wavelength selectivity. In thehologram optical element 34 having an interference fringe that causesdiffraction of the light with an incident angle greater than zerodegree, the wavelength selectivity is shorter in the directionperpendicular to the incident surface than in the direction parallelthereto (namely, shift of the diffraction wavelength resulting fromshift of the incident angle is smaller). To put it another way, theangle selectivity with respect to the shift of the incident angle to theinterference fringe is smaller in the direction perpendicular to theincident surface than in the Y-axis direction parallel to the incidentsurface. This is because, when the light with an incident angle entersthe interference fringe of the hologram optical element 34, the shift ofthe incident angle within the incident surface (YZ plane) is directlyrecognized as the angular shift of the incident angle, and therefore,directly affects the diffraction wavelength. However, the angular shiftin the direction perpendicular to the incident surface is recognized assmall angular shift of the incident angle, and therefore, its influenceto the diffraction wavelength is small.

Thus, if the light has entered the interference fringe at an angleshifted from a prescribed incident angle, the angular shift in thedirection parallel with the incident surface (Y-axis direction) causesgreater shift in diffraction wavelength than the angular shift in thedirection perpendicular to the incident surface (X-axis direction),under, the condition that those angular shift are same to each other(namely, the direction parallel with the incident surface exhibitsgreater wavelength selectivity).

Accordingly, when the optical pupil E is formed smaller in the Y-axisdirection in which the diffraction wavelength changes greatly, the rangeof changes in the diffraction wavelength is reduced, and therefore, thecolor nonuniformity on the optical pupil E can be reduced moreeffectively. Further, the observer can be provided with an image of highcolor purity, even if the optical pupil E is formed larger in thedirection perpendicular to the incident surface. It should be notedthat, as for light traveling outside the optical-axis incident surface,the incident surface of the light is slightly unparallel with theoptical-axis incident surface, but the angular shift in the directionperpendicular to the incident surface has a smaller impact on thediffraction wavelength, as described above. This prevents the colornonuniformity from being increased even when the above issue isconsidered with reference to the incident surface of the optical axis.

In the present embodiment, the diffusing direction of diffusing means 23is set to the X-axis direction, which is perpendicular to theoptical-axis incident surface of the hologram optical element 34. Bysetting the diffusing direction of the diffusing means 23 to the X-axisdirection being perpendicular to the optical-axis incident surface orthe direction almost parallel with the same, and by diffusing light inthe direction where the wavelength selectivity of the hologram opticalelement 34 is smaller, as described above, the optical pupil E isincreased in size in the above-mentioned direction, and the observer isprovided with an easy-to-view image, whereas the color nonuniformity iskept suppressed. Further, the optical pupil E is smaller in the Y-axisdirection parallel with the optical-axis incident surface than in theX-axis direction perpendicular to the optical-axis incident surface.This allows the light from the light source 21 to be convergedeffectively in the Y-axis direction and provides the observer with abright image.

As described above, the three light-emitting sections 21R, 21G, and 21Bof the light source 21 are arranged in the X-axis direction wherein theamount of diffusion is greater. This signifies that three light-emittingsections 21R, 21G, and 21B are arranged in the direction perpendicularto the optical-axis incident surface. The direction perpendicular to theincident surface corresponds to the direction in which the wavelengthselectivity in the hologram optical element 34 is small. Therefore, byarranging the three light-emitting sections 21R, 21G, and 21B in theX-axis direction, it allows the colors to be mixed in the directionwherein the optical pupil E can be increased in size. Even when thelight source 21 for emitting the three colors R, and B is used, theobserver can be supplied with a high-quality image of reduced colornonuniformity.

Embodiment 3

The following describes still another embodiment of the presentinvention with reference to the drawings. For purposes of description,the same portions as the aforementioned second embodiment will beassigned with the same numerals of reference, and will not be describedto avoid duplication.

FIG. 14 is an explanatory diagram showing the optical path of the imagedisplay device 11 unfolded optically in one direction. The presentembodiment is the same as the second embodiment except that the lightsource 21 is made of two light source groups 21P, 21Q.

The light source group 21P of the light source 21 is composed of a LEDwherein elements for R, G, and B are integrated into one body, includingthree light-emitting sections 21R₁, 21G₁, and 21B₁ emitting light ofthree colors of R, G, and B. Similarly, the light source group 21Q iscomposed of a LED wherein elements for R, and B are integrated into onebody, including three light-emitting sections 21R₂, 21G₂, and 21B₂emitting light of three colors of R, G, and B. To put it another way,the light source 21 contains two sets of three light-emitting sections(LEDs) that emit light of R, G, and B colors.

The light-emitting sections of each of the light source groups 21P, 21Qare arranged in the direction perpendicular to the optical-axis incidentsurface (YZ plane) of the hologram optical element 34. Further, theselight-emitting sections are arranged to be symmetric with respect to theaforementioned incident surface for each color (they are arrangedsymmetrically with respect to the optical axis of the ocular opticalsystem 31). To put it in more detail, this arrangement is made in such away that the light-emitting sections 21R₁, 21R₂ are plane-symmetric withrespect to the aforementioned incident surface. On the outside thereofin the X-axis direction, light-emitting sections 21G₁, 21G₂ are arrangedplane-symmetric with respect to the aforementioned incident surface.Still on the outside thereof in the X-axis direction, light-emittingsections 21B₁, 21B, are arranged plane-symmetrically with respect to theaforementioned incident surface. To put it another way, in the lightsource groups 21P, 21Q, light-emitting sections are arranged in theorder wherein the wavelength of the outgoing light is reduced as onegoes from the aforementioned incident-surface side toward the outside inthe X-axis direction.

As described above, light-emitting sections are arranged plane-symmetricwith respect to the aforementioned incident surface for each color. Thisarrangement ensures that the center of gravity of the total lightintensity obtained by the addition of the intensifies of the lightemitted from two light-emitting sections (for example, 21R₁ and 21R₂)for one and the same color, can be positioned within the plane ofsymmetry (within the aforementioned incident surface, and on the opticalaxis), for each of R, G, and B. Thus, the observer is provided with animage of reduced color nonuniformity at the center of the optical pupilE as a high-quality image of reduced aberration.

The surface serving as a center of the plane-symmetry of eachlight-emitting section may be a surface parallel with the aforementionedincident surface. To put it another way, the surface as the center ofthe plane-symmetry of each light-emitting section can be slightlymisaligned from the incident surface in the X-axis direction. In thiscase, the observer is provided with an image of reduced colornonuniformity approximately at the center of the optical pupil E.

Further, as described above, the hologram optical element 34 is designedto diffract the image light having wavelengths of 465±5 nm (light B),521±5 nm (light G), and 634±5 nm (light R) at the peak wavelength ofdiffraction efficiency and its half-value wavelength width. As describedabove, since the half-value wavelength width of diffraction efficiencyis the same for each color, the light having a longer wavelength has agreater angular selectivity (a smaller shift in incident angle withrespect to a change in wavelength). Thus, in each of the light sourcegroups 21P, 21Q, light-emitting sections are arranged in such an orderthat, as one goes from the side of the optical-axis incident surface tothe outside in the X-axis direction, the wavelength of the outgoinglight is shorter. This arrangement presents the observer with an imageof reduced color nonuniformity within the optical pupil E. The detailsof this arrangement will be described below.

Assume that λ is the wavelength of the peak diffraction efficiency, n isthe refractive index of the medium (interference fringe) of the hologramoptical element 34, h is the thickness of the medium, and θ is theincident angle. The relationship of λ=2nh cos θ holds among thesefactors. If, in light B with shorter wavelength and light R with longerwavelength, those wavelengths shift by the same amount of 5 nm, forexample, the percentage of the change in wavelength is 465/470 for lightB, and 634/639 for light R. To put it another way, the percentage of thechange in wavelength is smaller for the light R having a longerwavelength than for the light B having a shorter wavelength. Thus, theshift of the incident angle θ with respect to changes in wavelength issmaller (angular selectivity is greater) in the light R having a longerwavelength than in the light B having a shorter wavelength. Accordingly,if the wavelength widths of the R, and B of the light emitted from thelight source 21 are the same, the size of the optical pupil E formed bydiffraction with the hologram optical element 34 is smaller for thelight having a longer wavelength. It should be noted that the opticalpupil E is assumed to contain the entire range of the optical pupil foreach color.

In the meantime, the intensity of the light emitted from the LED (thelight-emitting sections) of the light source 21 is generally higher at aposition closer to the center, and is lower as one goes away from thecenter. The light-emitting sections are arranged to be approximatelyconjugate to the optical pupil in the Y-axis direction. In the X-axisdirection, however, the light-emitting sections are not conjugate withthe optical pupil, because the emitted light is diffused by thediffusing means 23. The position of highest intensity in the opticalpupil, however, is approximately equal to the position conjugate to theeach light-emitting section, if there is assumed to be no diffusingmeans 23.

Accordingly, the center of the pupil for the longer wavelength (light R)having a smaller optical pupil is located at the center of the opticalpupil E, and the center of the pupil for the shorter wavelength (lightB) having a greater optical pupil is located outside the center of theoptical pupil E. This arrangement ensures that the difference in theintensity due to the position of the pupil inside the optical pupil Ecan be reduced for each color. The following describes this mechanism inmore detail.

FIG. 15 is an explanatory diagram showing the relationship between theposition of the optical pupil E in the X-axis direction and the lightintensity. The light intensity is represented in relative values for thesame color. Further, the curves indicated by 21R₁, 21R₂, 21G1, 21G₂,21B₁, and 21B₂ in the drawing correspond to the beams of light emittedfrom the light-emitting sections 21R₁, 21R₂, 21G1, 21G₂, 21B₁, and 21B₂,respectively.

As described above, the angular selectivity of the hologram opticalelement 34 ensures that the optical pupil is smaller as the wavelengthis longer. As shown in FIG. 15, the light having a longer wavelengthexhibits a greater difference in intensity due to the position of thepupil (i.e., the difference in intensity between the center and edge ofthe optical pupil E is greater). Conversely, the light having a shorterwavelength has a greater size in the optical pupil E, and therefore,exhibits a smaller difference in intensity due to the position of thepupil.

Further, because the light-emitting section for emitting the light oflonger wavelength is arranged closer to the optical-axis incidentsurface, the position of higher light intensity is closer to the centerof the optical pupil E for the light having a longer wavelength.Conversely, the light-emitting section for emitting the light of shorterwavelength is arranged further away from the optical-axis incidentsurface. Accordingly, the position of higher light intensity is aroundthe optical pupil E.

To put it another way, the light of longer wavelength has a greaterdifference in intensity due to the position of the pupil. However, thelight-emitting sections are arranged in such an order that, as one goesfrom the optical-axis incident surface to the outside in the X-axisdirection, the wavelength of the outgoing light is reduced, to locatethe position of higher light intensity at a closer position to thecenter of the optical pupil E for the light of longer wavelength. Thisarrangement ensures that the difference in intensity due to the positionof the pupil, namely, reduce the difference in intensity between thecenter and edge of the optical pupil E, is reduced for the light oflonger wavelength. This arrangement provides the observer with an imageof reduced color nonuniformity over the overall area of the opticalpupil E (both the center and periphery of the pupil).

Further, since the light-emitting section for emitting the light oflonger wavelength is arranged closer to the optical-axis incidentsurface, the efficiency of utilizing the light on the optical pupil Ecan be improved. In the meantime, when the light-emitting section foremitting the light of shorter wavelength is arranged further away fromthe optical-axis incident surface, there is a smaller difference betweenthe peak intensity and peripheral intensity, which does not deterioratethe efficiency of utilizing the light significantly. Therefore, thedifference in intensity within the optical pupil E of each color isreduced and the luminance nonuniformity is also reduced.

The light-emitting sections of the light source groups 21P, 21Q arearranged in the X-axis direction in the descending order of diffusion bythe diffusing means 23 (the shorter the wavelength, the greater thediffusion). This further reduces the difference in intensity for eachcolor on the optical pupil E and also reduces the color nonuniformity.To be more specific, the observer is provided with an image of highcolor purity.

The above description uses an example wherein two sets of R, G, and Blight-emitting sections are provided, and the light source 21 is made upof the light source groups 21P, 21Q wherein each set is formed into anindividual package. However, each set need not always be formed into onepackage. The colors R, G, and B are mixed more effectively as thedistance of the light-emitting point is closer, with the result that abrighter image can be provided. Accordingly, in this respect, each setof the light source groups is preferably formed into one package,because the distance of each light-emitting section can be easilyreduced.

From the viewpoint of reducing the color nonuniformity, the imagedisplay device 11 of the present embodiment can be designed in thefollowing configuration. The light-converging lens 22 of the imagedisplay device 11 can be a cylindrical lens power-less in the directionwherein the light-emitting sections are arrayed (in the X-axisdirection), or a lens having negative power in the aforementionedarrayed direction (in the X-axis direction). In the former case, thepercentage of enlarging the distance among light-emitting sections of R,and B color is reduced. Accordingly, an image of reduced colornonuniformity can be obtained, with the degree of diffusion beingreduced. In the latter case, a bright image is obtained, with the colornonuniformity further reduced.

Embodiment 4

Referring to the drawings, the following describes still a furtherembodiment of the present invention. The same members as theaforementioned second or third embodiment will be assigned with the samereference numerals, and will not be described.

FIG. 16 is a cross sectional view showing the schematic configuration ofthe image display device 11 in the present embodiment. The image displaydevice 11 is the same as that of the second embodiment except that areverse side reflecting mirror 25 is installed instead of thelight-converging lens 22 of the second embodiment (FIG. 10).

The reverse side reflecting mirror 25 is an optical-path bending memberfor bending the optical path from the light source 21 to the displayelement 24, and is provided with a refracting surface 25 a andreflecting surface 25 b. The reflecting surface 25 b is formed, forexample, of a cylindrical mirror for converging the light from the lightsource 21 in the Y-axis direction. It can also be formed of anotherconcave mirror (spherical mirror, aspherical mirror, or axi-asymmetricconcave mirror). The refracting surface 25 a has a curvature similar tothat of the reflecting surface 25 b.

In this configuration, the light from the light source 21 is refractedby the refracting surface 25 a of the reverse side reflecting mirror 25to reach the reflecting surface 25 b, then, this light is reflected andis again refracted by the refracting surface 25 a. Then the light is ledto the diffusing means 23. The subsequent optical path is the same asthat of the second embodiment.

The reverse side reflecting minor 25 converges the light from the lightsource 21 in the Y-axis direction, sets the light source 21 to beconjugate with the optical pupil E in the Y-axis direction, and does nothave optical power in the X-axis direction. This procedure ensures easymixing of colors by the diffusing means 23, and provides an imagecharacterized by reduced color nonunifonmity, without increasing thecolor-to-color distance of the R, G, and B light-emitting sections ofthe light source 21 wherein separation occurs at pitches of 0.5 mm inthe X-axis direction.

The reverse side reflecting mirror 25 converges the incident light bythe optical power caused by reflection, and easily increases the opticalpower than the transparent lens. This allows the light from the lightsource 21 to be converged more in the Y-axis direction so that a brightimage can be supplied. Further, the incident light is converged by theoptical power caused by reflection, and therefore, chromatic aberrationis reduced. The same conjugation between the light source 21 and opticalpupil E can be used for different colors, so that effective use of coloris ensured.

As described above, the hologram optical element 34 has greaterwavelength selectivity in the Y-axis direction. In the meantime, thereverse side reflecting mirror 25 is provided with a refracting surface25 a in addition to the reflecting surface 25 b. The light from thelight source 21 is refracted by the refracting surface 25 a and isreflected by the reflecting surface 25 b. After that, this light isagain refracted by the refracting surface 25 a and is led to thediffusing means 23. This procedure allows the aberration to be reducedin such a way as to achieve approximate agreement between the peakwavelength of the light from the light source 21 and the diffractionpeak wavelength of the hologram optical element 34. This reduces thereduction in the diffraction efficiency resulting from wavelengthselectivity of the hologram optical element 34, and a bright image canbe provided by an effective use of light.

Further, the light is bent by the reverse side reflecting mirror 25 toilluminate the display element 24. This structure makes an effective useof the illumination optical path (whereby the illumination system can bedesigned in a compact structure), and reduces the size of the casing 20,with the result that an image display device 11 designed in a compactand lightweight structure is implemented.

Since the illumination optical path is bent by the reverse sidereflecting minor 25, the diffusing means 23 is placed close to thedisplay element 24 in the present embodiment, although the layout of thediffusing means 23 undergoes some restrictions. The following describesthe reason for this configuration.

The diffusing means 23 can be placed close to the light source 21. Inthis case, however, the optical path subsequent to diffusion of light bythe diffusing means 23 is long, and this reduces the amount of light ofthe light source 21 passing through the display element 24. Further, tomix R, and B colors, the degree of diffusion of the diffusing means 23must be increased. This results in poorer efficiency in the use oflight, and a less bright image is supplied.

Further, the diffusing means 23 can be placed close to the reverse sidereflecting mirror 25. However, the reverse side reflecting mirror 25 isinstalled in the form inclined with respect to the display element 24,and therefore, the efficiency of using the diffused light on thediffusing means 23 differs according to the position of the image in theY-axis direction of the display element 24, with the result thatluminance nonuniformity occurs in the Y-axis direction and the imagequality is deteriorated.

For the aforementioned reason, the diffusing means 23 is placed close tothe display element 24 in the present embodiment. To be more specific,the diffusing means 23 is installed on the optical path between thereverse side reflecting mirror 25 and display element 24. Thisarrangement allows the optical path to be bent by the reverse sidereflecting mirror 25, whereby a compact and lightweight structure isformed. At the same time, the display element 24 is allowed to makeeffective use of the diffusing means 23, whereby the image to beobserved is converted to a bright image free from color nonuniformity orluminance nonuniformity.

The reverse side reflecting mirror 25 can be provided with optical powernegative in the X-axis direction. In this case, there is a reduction inthe color-to-color distance of the R, and B light-emitting sectionswherein separation occurs at pitches of 0.5 mm in the X-axis direction.This ensures easy color mixing of the diffusing means 23 and provides animage characterized by reduced color nonuniformity.

The image display device preferably applicable to an HMD has beendescribed with reference to the embodiments. It should be noted that theimage display device of each of the embodiments is applicable to otherdevices such as a Head Up Display (HUD) as well.

It goes without saying that an image display device, HMD, and HUD can beimplemented by appropriate combination of the structures of theaforementioned embodiments.

INDUSTRIAL APPLICABILITY

The image display device of the present invention is applicable to theHMD and HUD, for example.

REFERENCE SIGNS LIST

-   -   1. Image display device    -   2. Light source    -   3. Diffusing means    -   3 a. Diffusing surface    -   3 b. Non-diffusing surface    -   4. Display element    -   5. Lens (ocular optical system)    -   11. Image display device    -   12. Supporting means    -   21. Light source    -   21R, 21R₁, 21R₂ Light-emitting sections (light-emitting diode)    -   21G, 21G₁, 21G₂ Light-emitting sections (light-emitting diode)    -   21B, 21B₁, 21B₂ Light-emitting sections (light-emitting diode)    -   22. Light-converging lens (Light-converging means)    -   23. Diffusing means    -   23 a. Diffusing surface    -   24. Display element    -   25. Reverse side reflecting mirror (optical-path bending member)    -   25 a Refracting surface    -   25 b Reflecting surface    -   31 Ocular optical system    -   32 Ocular prism (first transparent substrate)    -   33 Deflecting prism (second transparent substrate)    -   34 Hologram optical element    -   A Pixel    -   BM Light-shielding section    -   E Optical pupil

1. An image display device comprising: a point light source; a diffusingmember comprising a diffusing surface in which recesses and protrusionsare formed in one direction with a plurality of pitches, for diffusinglight traveling from the light source by the diffusing surface in theone direction and emitting the diffused light; a display elementcomprising a plurality of pixels and a light-shielding section formedaround the plurality of pixels, for displaying an image by modulatinglight traveling from the diffusing member with the plurality of pixels;and an ocular optical system for guiding light of an image travelingfrom the display element to an optical pupil, to make an observerobserve a virtual image of the image at the optical pupil, wherein theimage display device satisfies the following conditional expression (1),where Hw is an aperture width of the pixels of the display element in adiffusing direction of the diffusing member, Kw is a maximum pitch ofthe recesses and protrusions in the diffusing direction of the diffusingmember, f is a focal length of the ocular optical system, and δ is adistance from a display surface of the display element to the diffusingsurface of the diffusing member:Hw/f≦tan 1′≦Kw/(f+δ)  (1).
 2. The image display device of claim 1,wherein an aperture ratio Hw/Hp of the pixels of the display element inthe diffusing direction of the diffusing member satisfies the followingconditional expression (1A), where Hp is a pitch of the pixels of thedisplay element in the diffusing direction of the diffusing member:0.3≦Hw/Hp≦0.8  (1A).
 3. The image display device of claim 1, wherein thediffusing member comprises a diffusing surface in which recesses andprotrusions are formed in one direction with a plurality of pitchesranging from 1 μm to 20 μm.
 4. The image display device of claim 1,wherein the light source comprises light-emitting diodes emitting lightof three colors of red, green, and blue, respectively, and thelight-emitting diodes are arrayed in the diffusing direction of thediffusing member.
 5. The image display device of claim 4, wherein thelight source comprises two sets of light-emitting diodes for emittingred, green, and blue light, respectively, and the light-emitting diodesof each color are arranged symmetrically with respect to an optical axisof the ocular optical system, where the optical axis is an axisoptically connecting a center of a display region of the display elementand a center of the optical pupil.
 6. The image display device of claim1, wherein the distance δ from the display surface of the displayelement to the diffusing surface of the diffusing member is set suchthat a difference in visibility between a virtual image formed by theocular optical system of the diffusing member and a virtual image of animage displayed on the display element is 10 diopters or less.
 7. Theimage display device of claim 1, wherein the diffusing member isarranged such that the diffusing direction intersects with a long-sidedirection of a rectangular display region of the display element.
 8. Theimage display device of claim 1, wherein a diffusion degree of thediffusing member in the diffusing direction is ten or more times largerthan a diffusion degree of the diffusing member in a directionperpendicular to the diffusing direction.
 9. The image display device ofclaim 1, wherein an opposing surface to the diffusing surface in thediffusing member is a non-diffusing surface, and the diffusing member isarranged such that the diffusing surface faces the display element. 10.The image display device of claim 1, further comprising alight-converging member for converging light traveling from the lightsource, in a direction perpendicular to the diffusing direction of thediffusing member and for guiding the light to the diffusing member,wherein the light source and the optical pupil are arranged at conjugatepositions with respect to a direction perpendicular to the diffusingdirection of the diffusing member.
 11. The image display device of claim10, wherein the light-converging member has no-optical-power ornegative-optical-power in a direction parallel with the diffusingdirection of the diffusing member.
 12. The image display device of claim1, satisfying the following conditional expression (2), where Vw is anaperture width of the pixels of the display element in a directionperpendicular to the diffusing direction of the diffusing member:tan 1′≦Vw/f  (2).
 13. The image display device of claim 1, furthercomprising an optical-path bending member for bending an optical pathfrom the light source to the display element, wherein the diffusingmember is arranged in an optical path between the optical-path bendingmember and the display element.
 14. The image display device of claim13, wherein the optical-path difference bending member comprises arefracting surface and a reflecting surface, and after the refractingsurface refracts light traveling from the light source and thereflecting surface reflects the light, the refracting surface refractsthe light again to guide the light to the diffusing member.
 15. Theimage display device of claim 1, wherein the ocular optical systemcomprises a hologram optical element of a volume phase type and areflection type, and the hologram optical element diffracts and reflectslight of an image traveling from the display element to guide the lightto the optical pupil.
 16. The image display device of claim 15, whereinthe hologram optical element has axi-asymmetric positive optical power.17. The image display device of claim 16, wherein the diffusingdirection of the diffusing member is almost parallel with a directionperpendicular to an optical-axis incident surface of the hologramoptical element, where the optical axis is an axis optically connectinga center of a display region of the display element to a center of theoptical pupil.
 18. The image display device of claim 15, wherein thelight source comprises a light-emitting diode, and a wavelength at whicha diffraction efficiency of the hologram optical element is maximum anda wavelength at which an intensity of light emitted from the lightsource are almost the same.
 19. The image display device of claim 1,wherein the ocular optical system comprises a first transparentsubstrate for totally reflecting light of an image from the displayelement on a inside thereof, to guide the light to the optical pupil,and for transmitting outside light to guide the outside light to theoptical pupil.
 20. The image display device of claim 19, wherein theocular optical system further comprises a second transparent substratefor canceling refraction of the outside light caused in the firsttransparent substrate.
 21. A head-mounting display comprising: the imagedisplay device of claim 1; and a supporting member for supporting theimage display device in front of an eye of an observer.